This invention relates to silica flatting agents for coating compositions, particularly to silica flatting agents that settle softly, if at all, and are easily redispersed; to methods for preparing such flatting agents; and to coating compositions containing them.
The problem of hard settling of silica flatting agents in coating compositions is well known, and methods of treating silica to correct the problem have been proposed. U.S. Pat. No. 2,625,492 discloses a non-hard settling silica flatting agent prepared by treating an ammoniated silicofluoride. U.S. Pat. No. 3,607,337 describes a soft-settling silica which is impregnated with an inert wax or a polymer such as polyethylene by stirring a wax or polymer emulsion into an aqueous suspension of precipitated silica, filtering, drying, and grinding to a fine powder.
The treatment of silica with reactive organopolysiloxanes is also known, but the treatment of a silica flatting agent with a reactive organopolysiloxane has not been shown. U.S. Pat. Nos. 3,929,718, 3,377,311, 3,085,905, 3,132,961, 3,556,830, 3,649,588, 3,700,473, and 3,948,676 relate generally to the treatment of finely divided silica with organopolysiloxanes for use as filler in silicone rubber. U.S. Pat. No. 3,634,288 relates to the preparation of hydrophobic, polysiloxane-treated silica sols for use as antifoam agents in, for example, latex paints.
U.S. Pat. No. 3,377,311 discloses the treatment of silica with an organopolysiloxane having an end group reactive with the silica, and discloses the use of the treated silica as filler in silicone rubber.
Dow Corning Corporation Bulletin 22-203, dated 6/72, discloses that Dow Corning.RTM. 1107 fluid, a methyl hydrogen polysiloxane, may be used to render silica hydrophobic and free flowing. However, no use for such treated silica is disclosed. All of the above prior art have taught or suggested the requirement of completely coating silica particles with coating such as polysiloxane to obtain redispersion or good adhesion with rubber.